The term “patterning device” as here employed should be broadly interpreted as referring to any device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such a patterning device include:                a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired;        a programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from United States patents U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and        a programmable LCD array. An example of such a construction is given in United States patent U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which maybe fixed or movable as required.        
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally<1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from United States patent U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in United States patent U.S. Pat. No. 5,969,441 and PCT patent application publication WO 98/40791, incorporated herein by reference.
In a lithographic apparatus, the size of features that can be imaged onto a substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation of around 13 nm. Such radiation is termed extreme ultraviolet, also referred to as XUV or EUV, radiation. The abbreviation ‘XUV’ generally refers to the wavelength range from several tenths of a nanometer to several tens of nanometers, combining the soft x-ray and vacuum UV range, whereas the term ‘EUV’ is normally used in conjunction with lithography (EUVL) and refers to a radiation band from approximately 5 to 20 nm, i.e. part of the XUV range.
A radiation source for XUV radiation may be a discharge plasma radiation source, in which a plasma is generated by a discharge in a substance (for instance, a gas or vapor) between an anode and a cathode and in which a high temperature discharge plasma may be created by Ohmic heating by a (pulsed) current flowing through the plasma. Further, compression of a plasma due to a magnetic field generated by a current flowing through the plasma may be used to create a high temperature, high density plasma on a discharge axis (pinch effect). Stored electrical energy is directly transferred to the plasma temperature and hence to short-wavelength radiation. A pinch may allow for a plasma having a considerably higher temperature and density on the discharge axis, offering an extremely large conversion efficiency of stored electrical energy into thermal plasma energy and thus into XUV radiation. The geometry of devices, such as plasma focus, Z-pinch, hollow-cathode and capillary discharge sources, may vary, but in each of these types a magnetic field generated by the electrical current of the discharge drives the compression.
FIGS. 5A to 5E are included to provide background information about the construction and operation of such a discharge plasma radiation source. FIGS. 5A to 5E show schematically a discharge plasma radiation source comprising a single discharge element 540 of the Z-pinch hollow-cathode type. The discharge element 540 has cylindrical symmetry and comprises an anode 520 and a cathode 510 connected by an electrically insulating cylindrical wall 525. An aperture 530 is provided in the anode 520 on a central axis B for passing electromagnetic radiation from the discharge element 540. The hollow cathode 510 is provided with an annular aperture 511 around the central axis B, and is further provided with a large cavity 512 behind the aperture 511. The cavity 512 also has an annular configuration around central axis B, and the walls of the cavity are a part of the cathode 510. A discharge power supply (not shown) is connected to the anode 520 and cathode 510 to provide for a pulsed voltage V across the anode-cathode gap inside the discharge element 540. Further, a suitable gas or vapor, is provided by a discharge material supply (not shown) at a certain pressure p between the anode and cathode. Examples of suitable substances are xenon, lithium, tin and indium.
A discharge may take place at low initial pressure (p<0.5 Torr) and high voltage (V<10 kV) conditions, for which the electron mean free path is large compared to the dimension of the anode-cathode gap, so that Townsend ionization is ineffective. Those conditions are characterized by a large electrical field strength over gas or vapor density ratio, E/N. This stage shows rather equally spaced equipotential lines EP having a fixed potential difference, as is depicted in FIG. 5A.
The ionization growth is initially dominated by events inside the hollow cathode 510 that operates at considerable lower E/N, resulting in a smaller mean free path for the electrons. Electrons e from the hollow cathode 510, and derived from the gas or vapor within the cavity 512, are injected into the anode-cathode gap, a virtual anode being created with ongoing ionization, which virtual anode propagates from the anode 520 towards the hollow cathode 510, bringing the full anode potential close to the cathode 510, as is shown in FIG. 5B by unevenly distributed equipotential lines EP. The electric field inside the hollow cavity 512 of the cathode 510 is now significantly enhanced.
In the next phase, the ionization continues, leading to a rapid development of a high density plasma region inside the hollow cathode 510, immediately behind the cathode aperture 511. Finally, injection of an intense beam of electrons e from this region into the anode-cathode gap, also shown in FIG. 5B, forms the final breakdown channel. The configuration provides for a uniform pre-ionization and breakdown in the discharge volume.
FIG. 5C shows that a discharge has been initiated and a low temperature plasma 535 of the gas or vapor has been created in the anode-cathode gap. An electrical current will be flowing within the plasma from cathode 510 to anode 520, which current will induce an azimuthal magnetic field, having magnetic field strength H, within the discharge element 540. The azimuthal magnetic field causes the plasma 535 to detach from the cylindrical wall 525 and to compress, as is schematically depicted in FIG. 5C.
Dynamic compression of the plasma will take place, as further depicted in FIG. 5D, because the pressure of the azimuthal magnetic field is much larger than the thermal plasma pressure: H2/8π>>nkT, in which n represents plasma particle density, k the Boltzmann constant and T the absolute temperature of the plasma. Electrical energy stored in a capacitor bank (part of the discharge power supply, which is not shown) connected to the anode 520 and cathode 510 will most efficiently be converted into energy of the kinetic implosion during the full time of the plasma compression. A homogeneously filled constriction (plasma pinch) 550 with a high spatial stability is created.
At the final stage of plasma compression, i.e. plasma stagnation on the central, or discharge, axis B, the kinetic energy of the plasma is fully converted into thermal energy of the plasma and finally into electromagnetic radiation 560 having a very large contribution in the XUV and EUV ranges (as depicted in FIG. 5E).
It is generally acknowledged that a number of improvements are required before EUV production by gas discharge plasma can be considered suitable (production-worthy) for large scale production of devices, for instance integrated circuits. These include:                higher conversion efficiency. Current sources typically display a conversion efficiency (ratio of power-out at required wavelength to power-in) of approximately 0.5%, resulting in the majority of the input power being converted into heat;        efficient heat removal (cooling). Two components may be distinguished—peak heat load from plasma jets during discharge and average heat load due to repeated discharge. The area over which the heat can be spread is typically limited, and heat removal becomes critical as power levels and repetition rates are increased to achieve a production-worthy source. Overheating of one or more electrode surfaces may occur, affecting pinch size and position if the geometry of the electrodes changes (deformation); and        stable pulse timing and energy. For use with a projection lithography device, the source should produce a stable output during projection. This can be negatively influenced by, for instance, variations in EUV pulse timing (jitter), variations in pinch position and size, and variations in EUV pulse energy.        
Thus, it would be advantageous to provide a radiation source that provides production-worthy power levels and repetition rates without risk of overheating. Further, it would be advantageous to provide a radiation source having well-defined timing and well-defined energy of generated pulses (shots) of XUV radiation. It would also be advantageous to provide a radiation source that provides an enhanced conversion efficiency of electrical energy into radiation.